1. Field of the Invention
A method and apparatus for softening and dealkalizing water in a reverisble electrolytic cell for domestic, industrial and other applications.
2. Description of the Prior Art
The presence of calcium and magnesium ions (hardness) and carbonate and bicarbonate ions (alkalinity) in raw water is objectionable for many purposes, and in many systems that utilize water. It, therefore, is considered desirable and often necessary, to subject raw water containing such ions to softening and/or dealkalizing treatment prior to its use.
Perhaps the most common and least tolerable chemical impurities in raw water are the hardness-producing ions, namely, calcium and magnesium. They are responsible for the tenacious scale which deposits and builds up in boilers, pipes, condenser jackets, circulating systems, cooking utensils, and other equipment contacted by hot water. The presence of calcium and magnesium salts also adversely affects the taste of many food products, particularly canned foods. It long has been known that hard water causes scaling in household appliances and that it produces a curd in the presence of common soaps before a lather can be created, thus soiling kitchen utensils. This curd also imparts a somewhat grayish appearance to laundry in addition to being wasteful in the use of soap.
Perhaps the greatest objection to alkalinity in raw water comes from the operators of steam boilers. Under the influence of heat, bicarbonate and carbonate ions decompose to yield carbon dioxide which thereupon reacts with water to form carbonic acid that in turn, reacts with and corrodes steel and other ferrous alloys. In addition, the corrosion product, ferrous bicarbonate, deposits in and eventually fouls water lines.
There are other objections to alkalinity in raw water. For example, a high bicarbonate content often causes ice (e.g. in ice cubes frozen from alkaline raw water) to be cloudy and brittle. Bottled beverages and other food products prepared with the use of alkaline raw water tend to have their acidic contents neutralized by alkaline water thereby to become flat and tasteless.
One of the standard methods of treating boiler makeup water is by the hot-lime precipitation process. In the treatment of water by hot-lime-soda, the most common hot precipitation method, lime and soda ash are added to the raw water supply. The lime reacts with the calcium bicarbonate and soluble magnesium salts present in the raw water, precipitating insoluble calcium carbonate and magnesium hydroxide. The function of the soda ash, which is generally added in excess of the stoichiometric amount, is to precipitate the soluble calcium salts as insoluble carbonates. This method, while reasonably effective, has always possessed certain disadvantages. Thus, large space is required for equipment, and elaborate chemical feed controls must be maintained to meter lime and soda ash to the system. Moreover, both lime and soda ash are relatively expensive.
A well known and very effective method for removing hardness and/or alkalinity from raw water supplies is the ion exchange process, which relies on the replacement of calcium and magnesium ions in raw water by an equivalent number of sodium ions. Hard raw water is passed through a column of a sodium form of a cation exchange resin in which the objectionable calcium and magnesium ions are replaced by non-objectionable sodium ions. When the capacity of the resin for adsorbing calcium and magnesium ions in such manner is exhausted, the column is regenerated with a salt (sodium chloride) solution.
In the case of alkaline raw water, i.e. water containing carbonate and bicarbonate ions, it may be desirable to remove such alkalinity. Again, this can be done by ion exchange. A known chloride-anion exchange process relies on replacing the alkalinity in the water with chloride ions, using a strongly basic anion exchange resin in the chloride form. When the capacity of the resin to function has been exhausted, regeneration of the resin to the chloride form is necessary. This is done by passing a solution of salt (sodium chloride) through the resin column.
A disadvantage with all such ion exchange procedures, as just described, is that the beds of ion exchange materials become exhausted after a period of use and, therefore, must be removed and regenerated at frequent intervals. The regeneration of the softening and dealkalizing ion exchange resin process is done by treatment with a substantial amount of regenerant brine solution, about 10 to 12 pounds of sodium chloride per cubic foot of resin. This treatment is inconvenient, time-consuming, and relatively expensive. Furthermore, such conventional ion exchange processes are neither very effective nor economical, when the raw water to be treated contains more than about 500 ppm of total dissolved solids.